Molten metals to be cast have their own proper timings when they should be poured into a die. If molten metal is poured in a die at a time earlier than its proper pouring timing, its viscosity is too high to spread over the entire cavity in the die, so that articles cannot be cast with precision. On the other hand, if the metal is poured later than the proper pouring timing, the casting temperature is so high that the metal may be evaporated, oxidized or degraded in composition. In addition, when the metal is poured into the die, it may stick to the die because of its high temperature. Like this, the timing of pouring molten metal into the die is critical to the quality of cast articles.
Conventionally, the time at which a molten metal should be poured into a die is determined by artisans, who monitors, by eyes, the metal being melted for minute vibrations, flow, deformation, glow, color etc. of the metal, to determine when the viscosity of the entire molten metal has decreased to a viscosity suitable for pouring the metal into the die.
The proper timing of the pouring of a metal into a die is correlated to the surface temperature of the molten metal. Therefore, it has been proposed to use an infrared radiation thermometer for measuring the surface temperature of a mass of molten metal to time the pouring of the metal. It is, however, very hard to detect an accurate surface temperature of a molten metal mass with an infrared radiation thermometer because of various reasons including the following ones. First, the amount of infrared radiation emitted differs from metal to metal. In addition, for a particular metal, the surface state of the molten metal mass changes from time to time, so that the amount of infrared radiation varies from time to time, too. Furthermore, from the time at which the metal starts melting and its viscosity starts decreasing, metal films, such as an oxide film, are formed to partly cover the surface of the molten metal mass and move on the surface, which causes the amount of emission of infrared radiation detected by the thermometer to randomly vary. Also, some metals may evaporate, and the evaporated metal gas and other gas may absorb or attenuate the emitted infrared light.
Fresh metal is not always used in casting, but metal obtained by cutting off unnecessary portions of a completed cast article may be recycled. Such recycled metal has a thick oxide film on its surface, which prevents detection of correct surface temperature of the molten metal. In addition, since an infrared radiation thermometer measures the temperature only at a small point on the surface of the molten metal mass, it is not possible to know the temperature of the molten metal as a whole. In other words, it is difficult to determine when the whole molten metal attains its proper pouring temperature, with the viscosity decreased to an appropriate value.
For the reasons as above stated, when an infrared radiation thermometer is used to determine the surface temperature of molten metal, a large error may result in measured temperature, which, in turn, may result in erroneous determination of the timing of pouring of the metal into a die. Thus, an infrared radiation thermometer is not always useable to precisely time the pouring of various metals under various melting conditions.
Another possible method to determine the optimum time for pouring may be to compare the shape of a mass of metal exhibited when it is heated and melted to flow with the shape of the mass of the metal when it is solid. However, this method is not applicable to some metals and recycled metals since they have a thick or hard oxide film on their surfaces, and, therefore, the shape or appearance changes only little even when the interior has melted and liquefied enough. This may cause the metals to be heated more than necessary, leading to defective casting.
Another problem in prior art is that when a plurality of solid lumps of metal are placed in a vessel for melting, they may melt in different times and in different ways, and, therefore, it is not possible or difficult to determine when all the metal lumps have melted into a uniform molten mass only from shape or appearance changes.
Because of the problems described above, it was very difficult to realize a reliable automated casting machine which can properly operate for different melting conditions.
An object of the present invention, therefore, is to provide an apparatus for determining a proper timing of pouring molten metal and a casting machine with such pouring timing determination apparatus.